Many automotive vehicles with internal combustion engines use mass air flow meters that are located upstream of the internal combustion engine to measure the amount of air flowing into the engine. Responsive to the mass air flow information, the vehicle's engine controller controls fuel flow into the engine to reduce pollution emission levels, increase fuel economy and increase engine performance. To optimize system performance, it is desirable that the mass air flow meter respond quickly enough to supply information representative of the measure of mass air flow into each cylinder of the engine.
One challenge that occurs in attempting to measure mass air flow into the engine is especially prevalent in four cylinder engines. This challenge is air pulses in the intake duct caused by the engine valving during intake and exhaust strokes. Four cylinder engines can generate oscillations of significant amplitude in the intake air flow and engine valve overlap can cause brief periods of flow out of the cylinder intake manifold, causing brief periods of reverse flow in the intake ducts. This bi-directional mass air flow pulsation can cause significant mass air flow meter errors if the mass air flow meter and/or the measurement technique are insensitive to flow direction. Thus, it is advantageous to have a bi-directional mass air flow sensing device especially to control four cylinder engine fueling using mass air flow sensing strategies.
A mass air flow sensor suitable for batch fabrication and associated control circuitry have been described in U.S. Pat. Nos. 4,576,050, 4,713,970, 4,782,708, 5,086,650, and 5,263,380, all assigned to the assignee of this invention.
FIGS. 1 and 2 illustrate schematically side and top views, respectively, of a typical bi-directional mass air flow sensing device according to these prior patents. The sensing device 10 is centered on a planar substrate such as silicon chip 9 and consists of a central heater 12 and two temperature sensitive resistors (thermistors) 14, 16 located equidistantly upstream and downstream from the heater 12. The two thermistors 14, 16 are at equal temperature in zero-flow conditions but are at different temperatures when fluid flows past sensing device 10. More particularly, the electronic circuit 18 causes heater 12 to generate heat that propagates more toward the downstream thermistor 14, 16 than the upstream thermistor 16, 14 resulting in a positive temperature difference between the downstream and upstream thermistors 14, 16. The sensor output is proportional to the instantaneous temperature difference between thermistors 14 and 16 and increases monotonically with flow magnitude. A negative difference between the downstream and upstream thermistors 14 and 16 occurs during reverse flow conditions. Thus the sensing device 10 is responsive to flow in both the direction indicated by arrow 20 and the reverse direction indicated by arrow 22.
Referring now to FIG. 3, the sensing device 10 is positioned in a duct with the sensing elements 12, 14 and 16, aligned orthogonal to the fluid flow axis 15 and the device's planar surface 13 parallel to the flow axis 15. In this orientation, the tangential component of fluid flow velocity within the boundary layer near the sensing surface varies in relation to the distance from the surface 13 and, close to the sensing elements 12, 14 and 16, is much less than the free stream velocity through the duct. The leading edge 17 of the sensing device 10 perturbs the flow. The flow perturbations 32 continue across the device 10 and are incident on the sensing elements 12, 14, 16 causing recirculation in the regions of the elements 12, 14 and 16 taking measurements. These and other flow phenomena may cause instability in the boundary layer thickness above the sensing location on the device 10, even in constant flow conditions. This impairs the precision of the flow measurement signal provided by the device 10 since the measurement signal may be a non-representative sampling of the flow through the duct. As a result the sensor output has a high noise content.
The above-mentioned U.S. Pat. No. 5,086,650 teaches that limitations associated with poor signal to noise ratio for the flow sensors similar to FIG. 3 may be overcome by tilting the sensing surface of the device into the flow as shown in FIG. 4. The angle 42 between the plane of the sensing device 10 and the flow axis 40 is typically in the range of 5.degree.-10.degree.. This orientation of the device 10 reduces the effects of turbulence caused by the leading edge of the device, as illustrated by the smooth flow line, reference 44.
Also by tilting the device 10 as shown in FIG. 4, the signal magnitude increases for all forward flow values because the orientation of the device 10 in the flow path causes compression of the air above the device surface, thinning the boundary layer so that the boundary layer remains more constant over the device and eliminates turbulence caused by the leading edge of the device.
The '650 patent illustrates that boundary layer thickness and stability is a critical parameter governing the performance of the mass air flow sensor. A limitation of the configuration shown in FIG. 4, however, is that the device when so oriented cannot be used not bi-directionally. However, FIG. 5, which is also shown in the '650 patent, illustrates how the tilted orientation of the same device 10 can be used to provide bi-directional air flow sensing. The sensing device 10 is mounted on a support 50 at a corner 54 of an elbow in the duct housing 51. Duct portions 52 and 56 branch off from the corner 54 at angles of approximately 15.degree. from each other. This allows flow of air through the duct 51 to be incident on the device 10 at an angle to the device surface from either direction 58 or direction 60. The configuration in FIG. 5 allows bi-directional flow measurement taking advantage of the tilting orientation of the sensor with respect to flow to reduce boundary layer thickness above the sensing elements.